The present invention relates to an amplifier and, more particularly, to a transistor direct-coupled amplifier for amplifying a DC balanced signal.
In general, despite the many types of monolithic ICs known as wide-range amplifiers, when these ICs are used as wide-range amplifiers without matching their characteristics, they may not be suitable for wide-range amplifying applications because they offer no degree of design freedom. In contrast to this, in an amplifier constituted by a combination of transistor circuits, a circuit can be designed to match the application purpose. Therefore, this latter type of amplifier is frequently used in a video circuit and the like due to its high degree of design freedom.
In a conventional direct-coupled amplifier, in order to effectively use the dynamic range of an output voltage to reach a power source voltage, when an operating point is set to be about 1/2 the power source voltage, variable resistors are used in a bias circuit and adjusted to absorb variations in other transistors and resistors.
In the conventional direct-coupled amplifier, when the operating point is set to be about 1/2 the power source voltage, some of the resistors of the bias circuit must be changed into variable resistors VR11 to absorb variations in other transistors and resistors, thereby posing some problems in the manufacture of the amplifier. In addition, after the variable resistors are adjusted, the amplifier has no countermeasure against other changes over time, such as a change in power source voltage, and a change in temperature.
Especially, since a circuit having a high voltage gain has a small input signal level, the circuit is sensitive to a change in DC operating point caused by these changes, a signal clamping distortion is disadvantageously generated caused by a shift in the operating point. In order to prevent this, an output level must be set to be low, or a power source voltage must be increased. These limitations are drawbacks in use of the circuit.